ALS is one of multiple degenerative motor neuron diseases that are clinically defined, based on the involvement of upper and/or lower motor neurons. ALS is the most common form of motor neuron disease. The diagnosis of ALS is based upon clinical criteria that include the presence of upper motor neuron and lower motor neuron signs, progression of disease, and the absence of an alternative explanation. There is no single diagnostic test that can confirm or entirely exclude the diagnosis of motor neuron disease.

Practically speaking, clinicians make the diagnosis of ALS when it is suggested by the history and physical examination, supported by electrodiagnostic studies, and not excluded by neuroimaging and laboratory studies.

History — The diagnosis of ALS is suggested when there are progressive symptoms consistent with upper and lower motor neuron dysfunction that present in one of four body segments (cranial/bulbar, cervical, thoracic, and lumbosacral) followed by spread to other segments over a period of months to years. The course is not relapsing and remitting, but rather is insidiously progressive. Involuntary weight loss and muscle wasting unrelated to nutrition may also occur.

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